1. Field of the Invention
This invention relates to a horizontal output circuit for correcting a pincushion distortion and more particularly to improvements made to correct a horizontal distortion produced by a secondary side load fluctuation of a flyback transformer.
2. Related Art Statement
In the deflection of a picture tube or cathode ray tube (CRT) in an ordinary color television receiver, on the basis of the noncoincidence of the curvature center and deflection center of the fluorescent screen, the raster on the fluorescent screen will be distorted in a pincushion fashion.
The pincushion distortion is corrected by increasing the horizontal amplitude as the vertical scanning center is approached. That is to say, it is corrected by parabolically amplitude-modulating the envelope of the horizontal deflecting current in the vertical cycle.
FIG. 1 shows an example of a horizontal output circuit having a conventional pincushion distortion correcting function. In the formation of this circuit, a driving pulse of a horizontal cycle is input into the base of a horizontal output transistor Tr, a damper diode D and resonant capacitor Cr are connected in paralled between the collector and emitter and further a series circuit of a horizontal deflecting coil Ly, S-distortion correcting capacitor (direct current blocking capacitor) Cs and secondary coil LH of a transformer T.sub.2 is connected in parallel. The collector of the transistor Tr is connected to a direct current source E through the primary coil L.sub.0 of a transformer T.sub.1. A current source decoupling capacitor C is connected in parallel with the direct current source E. The above mentioned transformer T.sub.2 is a pincushion distortion correcting transformer and a parabolic driving current of a vertical cycle is flowed through its primary coil L.sub.v. By the way, this driving current is obtained by integrating the voltage of a vertical deflecting coil in an integrating circuit,
shaping the waveform and feeding the voltage to the coil L.sub.v. The above mentioned transformer T.sub.1 is a flyback transformer and its secondary side winding feeds a high voltage to a CRT anode through a rectifying circuit not illustrated.
In the circuit of the above mentioned FIG. 1, when a parabolic current of a vertical cycle is flowed through the above mentioned coil L.sub.v, the inductance of a coil L.sub.H connected in series with the horizontal deflecting coil L.sub.y will parabolically vary and the envelope of the horizontal deflecting current will be able to be parabolically amplitude-modulated in the vertical cycle.
However, in the circuit using the correcting transformer in FIG. 1, in case a large correction amount is required, the variation of the inductance of the coil L.sub.H will have to be made large, therefore the high voltage ripple obtained by the rectification in the secondary side high voltage rectifying circuit of the flyback transformer T.sub.1 will increase and a raster distortion by this ripple will be produced. Further, in the above mentioned circuit, there are such defects that the correcting transformer will be saturated with the horizontal deflecting current, the horizontal linearity will deteriorate and the correction amount will become non-uniform depending on the horizontal direction.
Further, in the circuit shown in FIG. 1, in case a white peak signal is received as a video signal, the load current on the secondary side of the flyback transformer T.sub.1 will increase and, under its influence, a current iL will flow on the primary side and will vary the horizontal deflecting current. FIG. 2 shows an example of a figure distorion by the above mentioned variation of the horizontal deflecting current. In the cross pattern by a white signal, the vertical line after the white horizontal line is reproduced is bent in the horizontal direction. Also, by the reduction of the secondary side high voltage, the rectangular picture image to be displayed on the picture surface will be distorted to be trapezoidal as shown in FIG. 3.
Further, in the circuit in FIG. 1, the pincushion distortion in the horizontal (right and left) direction is corrected by a constant parabolic information. However, depending on the CRT, the distortions outside and inside the Picture surface may be different and the inside distortion may be larger. In such case, the distortion will be more remarkable inside the picture surface. However, in the circuit in FIG. 1, such distortion can not be coped with.
On the other hand, there is such conventional example of a horizontal output circuit wherein a distortion is corrected without using a pincushion distortion correcting transformer as is shown in FIG. 4. This horizontal output circuit is mentioned in U.S. Pat. No. 3,906,305. In FIG. 4, a pulse of a horizontal cycle is input into the base of a horizontal output transistor T.sub.r, a series circuit of the first and second damper diodes D.sub.1 and D.sub.2 is connected in parallel between its collector and emitter, further first and second resonant capacitors C.sub.r and C.sub.r ' are connected in parallel with the respective diodes D.sub.1 and D.sub.2, a series circuit of a horizontal deflecting coil L.sub.y and S-distortion correcting capacitor C.sub.s is connected in parallel with the capacitor C.sub.r a series circuit of a coil L.sub.1 and capacitor C.sub.1 is connected in parallel with the capacitor C.sub.r ', the voltage from a d.c. voltage source E is to be given to the collector of the transistor T.sub.r through the primary coil L.sub.0 of the transformer T.sub.1 and a modulating source A is connected in parallel with the above mentioned capacitor C.sub.1. The modulating source A is to feed a vertical sawtooth wave signal fed as an input to the base of a transistor T.sub.r ' through a driving stage D.sub.r ' to obtain a parabolically varying control signal between the collector and emitter of the transistor T.sub.r '. By applying this control signal to both ends of the capacitor C.sub.1, the pincushion distortion of the horizontal deflecting current i.sub.y is corrected. By the way, a current source decoupling capacitor C is connected in parallel with the d.c. voltage source E.
In the circuit in FIG. 4, at the time of starting scanning, the diodes D.sub.1 and D.sub.2 will conduct. During the scanning period, the capacitor C.sub.s will become a power source of the deflecting coil L.sub.y and the capacitor C.sub.1 will become a power source of the coil L.sub.1 and therefore, by the conduction of the diodes D.sub.1 and D.sub.2, a sawtooth wave current will flow through the coils L.sub.y and L.sub.1 and the current i.sub.y flowing through the coil L.sub.y will be a deflecting current.
Before the middle of the scanning period is reached, a control signal (on pulse) will be fed to the base of the transistor T.sub.r to conduct the transistor T.sub.r. Thereby, substantially in the middle of the scanning cycle, the coil current will reverse the direction. In case the current i.sub.y is larger than the current i.sub.1 flowing through the coil L.sub.1, the current i.sub.y will flow through the transistor T.sub.r and its difference current i.sub.y -i.sub.1 will flow through the diode D.sub.2. In this case, the diode D.sub.1 will not conduct. In case the current i.sub.1 is larger than the current i.sub.y, a current amount corresponding to the current i.sub.1 will flow through the transistor T.sub.r and its difference current i.sub.1 -i.sub.y will flow through the diode D.sub.1. In this case, the diode D.sub.2 will not conduct.
At the time of the end of the scanning peiod, the transistor T.sub.r and conducting diode D.sub.1 or D.sub.2 will be cut off and a blanking period will be entered. Therefore, the current which has been flowing to the transistor T.sub.r will now flow into resonant capacitors C.sub.r and C.sub.r ' and a substantially sinusoidal wave-like resonant voltage will be generated between both terminals. The moment this voltage again becomes zero, the diodes D.sub.1 and D.sub.2, will simultaneously begin to conduct and a new scanning period will begin.
In the above mentioned circuit, the high voltage ripple on the secondary side of the flyback transformer is controlled by the flowing of the above mentioned difference current i.sub.1 -i.sub.y. Now, in the circuit in FIG. 4, if the alternating current flowing through the primary side of the flyback transformer T.sub.1 is represented by i.sub.p, the direct current is represented by i.sub.in and the alternating current flowing through the coil L.sub.1 is represented by i.sub.1, the current i.sub.p +i.sub.1 +i.sub.in will represent a deflecting current i.sub.y as shown in FIG. 5(a). Here, it is considered that a white square wave video signal is received and a white peak current flows through the secondary side of the flyback transformer T.sub.1. Then, the direct current i.sub.in flowing through the primary side of the transformer T.sub.1 will be increased by the above mentioned white peak current. At this time, in case, at the time t.sub.1 before the time t.sub.2 when such on-pulse as is shown in FIG. 5( b) enters the base of the horizontal output transistor T.sub.r, the damper diode D.sub.1 (D.sub.2) is "OFF" and the deflecting current becomes zero, such discontinuous pulse e.sub.1 will be generated as shown in FIG. 5(c) in both end voltages V.sub.cr ' of the second resonant capacitor C.sub.r '. At this time, the pulse e.sub.1 will operate to reduce the average value V.sub.cs of both end voltages V.sub.cs, because V.sub.cs +V.sub.cr '=E. Therefore, when a white square wave signal is input into the CRT, from above a certain signal level, the voltage V.sub.cs will gradually reduce in response to the horizontal output circuit and the rectangular picture image to be displayed on the picture surface will be distorted to be inverted trapezoidal as shown in FIG. 6. Also, in the circuit shown in FIG. 4, it is necessary that, on the first circuit network consisting of the parallel connections of the L.sub.y, C.sub.r and D.sub.1 and the second circuit network consisting of the parallel connections of the L.sub.1, C.sub.r ' and D.sub.2, the respective blanking periods should end simultaneously and the constants shold be selected so that L.sub.y .multidot.C.sub.r =L.sub.1 .multidot.C.sub.r ' and it is difficult to determine the circuit constants so as to satisfy this relation and the picture surface linearity correcting amount (pincushion distortion correcting amount).
By the way, a horizontal output circuit whereby a pincushion distortion is corrected without using a correcting transformer is suggested in each of U.S. Pat. Nos. 4,254,365, 4,733,141 and 4,482,846 but, by either of the suggestions, the trapezoidal distortion at the time of receiving the above described white peak signal has not been able to be corrected.